The present invention relates to a liquid crystal display device having a high picture quality and in which residual image is substantially eliminated.
In a conventional liquid crystal display device, two facing transparent electrodes formed respectively on each of two substrates are used as the electrodes for driving the liquid crystal layer. In such a device, a display method represented by a twisted nematic display has been adopted, wherein the crystal display operates by being supplied with an electric field having approximately a vertical direction to the substrate boundary planes. On the other hand, in arrangements wherein the electric field has approximately a parallel direction to the substrates, methods utilizing a pair of comb-like electrodes are disclosed, for example, in JP-B-63-21907 and WO91/10936. In these cases, the electrodes are not necessarily transparent, since opaque metallic electrodes having high conductivity are used. However, the above-mentioned publications do not provide any teachings concerning liquid crystal material, oriented film and insulating film, which are necessary for obtaining high picture quality when driving the display system, in which the electric field is supplied to the liquid crystal in an approximately parallel direction to the substrate plane (hereinafter referred to as an in-plane switching system), with an active matrix driving method or a simple matrix driving method.
When a character or a drawing is displayed in a display plane, an image of the character or the drawing remains for a while in the display plane even after erasing, and sometimes it causes an uneven display known as an afterimage. The afterimage is a common problem which causes deterioration of image quality for both the display method wherein the electric field is supplied in a perpendicular direction and the display method involving the in-plane switching system. Especially, in case of the in-plane switching system, the afterimage is generated more easily than the case wherein the electric field is generated perpendicularly to the substrate plane.
The object of the present invention is to provide a liquid crystal display device providing a high picture quality and in which the problems of residual image is substantially eliminated.
In order to solve the above described problems, the inventors realized the invention explained hereinafter.
As a first feature, a liquid crystal display (hereinafter called a liquid crystal display device of an in-plane switching system) is provided, wherein:
display picture elements are composed of electrodes on a substrate;
an orienting film for a liquid crystal layer is formed on the substrate directly or via an insulating layer;
the substrate is arranged so as to face another transparent substrate on which another orienting film is formed;
the liquid crystal layer is held between the above two substrates;
the electrodes are formed so as to generate an electric field which is applied to the liquid crystal layer in a direction parallel to the substrate;
the electrodes are connected to external control means; and
a polarizer for changing the optical characteristics of the liquid crystal layer is provided, wherein
a brightness recovering time of no greater than five minutes is obtained, where the brightness recovering time is the time until the brightness of a displayed portion that has been displayed for 30 minutes and is turned off returns to the background brightness.
In the liquid crystal display device of an in-plane switching system, the display picture elements are composed of scanning signal electrodes and image signal electrodes. Further, provision of picture element electrodes and active elements are desirable, but, this condition is not essential to the present invention.
Here, the orienting film refers to a film having a function to orient the liquid crystal. The insulating film refers to a film for electrically insulating, however, the film concurrently can have a function to protect an electrode.
As a second feature of this invention, a liquid crystal display device of an in-plane switching system is provided, wherein
the necessary time for recovering brightness is less than five minutes, and
respective products (xcex5rxcfx81)LC, (xcex5rxcfx81)AF, and/or (xcex5rxcfx81)PAS) of a specific dielectric constant xcex5r and a specific resistivity xcfx81 of the liquid crystal layer (abbreviated as LC), the orienting film (abbreviated as AF), and/or the insulating film (abbreviated as PAS) are in a range of 1xc3x97109 xcexa9.cm-8xc3x971015 xcexa9.cm.
Here, the dielectric constant of the liquid crystal layer xcex5r is an average dielectric constant expressed by the following equation:
xcex5r=(xcex5||+2xcex5xe2x8axa5)/3
where, xcex5|| is the dielectric constant in a molecular major axis direction, and xcex5xe2x8axa5 is the dielectric constant in a molecular minor axis direction.
When xcex5rxcfx81 is less than 1xc3x97109 xcexa9.cm, the device can not maintain its insulating property and a sufficient voltage keeping rate.
As a third feature of this invention, a liquid crystal display device of an in-plane switching system is provided, wherein
the necessary time for recovering brightness is less than five minutes, and
respective values of surface resistance of the orienting film and/or the insulating film are in a range of 3xc3x971011 xcexa9/xe2x96xa1-2.5xc3x971018 xcexa9/xe2x96xa1.
When the values of surface resistance are less than 3xc3x971011 xcexa9/xe2x96xa1, the device can not maintain its insulating property and voltage keeping rate.
As a fourth feature of this invention, a liquid crystal display device of an in-plane switching system is provided, wherein
respective products ((xcex5rxcfx81)LC, (xcex5rxcfx81)AF, and/or (xcex5rxcfx81)PAS) of a specific dielectric constant xcex5r and a specific resistivity xcfx81 of the liquid crystal layer, the orienting film, and/or the insulating film mutually have an approximately similar value.
As a fifth feature of this invention, a liquid crystal display device similar to the fourth feature is provided, wherein
the respective products are in a range of 1xc3x97109 xcexa9.cm-8xc3x971015 xcexa9.cm.
As a sixth feature of this invention, a liquid crystal display device of an in-plane switching system is provided, wherein
a ratio of the maximum value to the minimum value of respective products ((xcex5rxcfx81)LC, (xcex5rxcfx81)AF, and/or (xcex5rxcfx81)PAS) of a specific dielectric constant xcex5r and a specific resistivity xcfx81 of the liquid crystal layer, the orienting film, and/or the insulating film is equal to or greater than 1 and equal to or less than 100.
As a seventh feature of this invention, a liquid crystal display device of an in-plane switching system provided, wherein
respective products ((xcex5rxcfx81)LC, (xcex5rxcfx81)AF, and/or (xcex5rxcfx81)PAS) of a specific dielectric constant xcex5r and a specific resistivity xcfx81 of the liquid crystal layer, the orienting film, and/or the insulating film have a relationship expressed by the following equations (1) to (3).
0.1xe2x89xa6(xcex5rxcfx81)LC/(xcex5rxcfx81)AFxe2x89xa610xe2x80x83xe2x80x83(1)
0.1xe2x89xa6(xcex5rxcfx81)LC/(xcex5rxcfx81)PASxe2x89xa610xe2x80x83xe2x80x83(2)
0.1xe2x89xa6(xcex5rxcfx81)AF/(xcex5rxcfx81)PASxe2x89xa610xe2x80x83xe2x80x83(3)
As an eighth feature of this invention, a liquid crystal display device of an in-plane switching system is provided, wherein
the sum of the film thickness of the orienting film and the insulating film on the substrate 1 is in a range 0.5-3 xcexcm.
As a ninth feature of this invention, a liquid crystal display device according to any of the first to eighth features is provided wherein the device is provided with
an input means for information;
a means for calculating or processing the information;
a device for outputting the calculated or processed information;
a memory device; and
an internal power source.
In the liquid crystal display device of the present invention, the thickness of the insulating film is preferably in a range of 0.4-2 xcexcm.
Further, in the liquid crystal display device of the present invention, the orienting film is preferably made of an organic material, and the insulating film is preferably made of an inorganic material. Furthermore, the orienting film is preferably made of an organic material, and the insulating film preferably has a double layer structure made of an inorganic material and an organic material.
Further, in the liquid crystal display device of the present invention, the orienting film is preferably made of an organic material and the insulating film is preferably made of an inorganic material, and the orienting film made of an organic material is preferably thicker than the insulating material made of an inorganic material.
Further, both of the orienting film and the insulating film are preferably composed of an organic material, and both of the orienting film and the insulating film are preferably composed of the same material. Furthermore, one side of a plane of the orienting film which abuts the liquid crystal is flat.
In order to realize a color display having a high picture quality, a color filter is preferably provided on either one of the substrates, and an insulator is preferably inserted between the color filter and the liquid crystal layer. Further, a film having a function to flatten steps on the color filter is preferably composed of an organic material, and a film composed of an inorganic material is preferably formed on the film composed of organic material. Furthermore, the orienting film is preferably formed on the substrate having a color filter by the intermediary of a layer composed of inorganic material.